Method for densification and spheroidization of solid and solution precursor droplets of materials using microwave generated plasma processing

ABSTRACT

A method for processing feed material to produce dense and spheroidal products is described. The feed material is comprised of powder particles from the spray-drying technique or solution precursor droplets from ceramic or metallic materials. The feed material is processed using plasma generated from a microwave. The microwave plasma torch employed is capable of generating laminar flow during processing which allows for the production of spheroidal particles with a homogenous materials distribution. This results in products having improved thermal properties, improved corrosion and wear resistance and a higher tolerance to interface stresses.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 13/675,541 filed on Nov. 13, 2012, which publishedas U.S. Publication No. 2014/0131906 on May 15, 2014.

TECHNICAL FIELD

The present invention is generally directed to a method for taking feedmaterial and processing the feed material to produce dense andspheroidal products. The feed material is comprised of powder particlesfrom the spray-drying technique or solution precursor dropletscontaining ceramic or metallic materials. More particularly, the presentinvention is directed to a method which uses a microwave plasma torchcapable of generating a laminar flow pattern during materials processingto produce dense and spheroidal products. The laminar flow in anaxisymmetric hot zone with a uniform temperature profile within thetorch allows for the production of uniform spheroidal particles with ahomogenous materials distribution, which leads to final productspossessing superior characteristics.

BACKGROUND OF THE INVENTION

One of the most important aspects of preparing industrial powders is thespheroidization process, which transforms powders produced by spraydrying and sintering techniques, or angular powders produced byconventional crushing methods, into spheres. Spheroidized particles aremore homogenous in shape, denser, much less porous, provide higherflowability, and possess lower friability. These characteristics makefor powders that are superior for applications such as injectionmolding, thermal spraying of coatings and provide parts having near netshapes.

Current spheroidization methods employ thermal arc plasma described inU.S. Pat. No. 4,076,640 issued Feb. 28, 1978 and radio-frequencygenerated plasma described in U.S. Pat. No. 6,919,527 issued Jul. 19,2005. However, these two methods present limitations which result fromthe characteristics of the radio-frequency plasma and the thermal arcplasma.

In the case of thermal arc plasma, an electric arc is produced betweentwo electrodes and is then blown out of the plasma channel using plasmagas. Powder is then injected from the side, perpendicular or at anangle, into the plasma plume, where it gets exposed to the hightemperature of the plasma and is collected as spheres in filters duringsubsequent processing. An issue with thermal arc plasma is that the hightemperature of the electrodes leads to erosion of the electrodes, whichleads to contamination of the plasma plume with the electrode material,resulting in the contamination of the powders to be processed. Inaddition, the thermal arc plasma plume has an inherently uneventemperature gradient, and by injecting powder into the plasma plume fromthe side, the powder gets exposed to an uneven temperature gradient thatleads to the production of particles that are not homogenous in size,density or porosity.

In the case of radio frequency plasma spheroidization, the plasma isproduced in a dielectric cylinder by induction at atmospheric pressure.Radio frequency plasmas are known to have low coupling efficiency of theradio frequency energy into the plasma and a lower plasma temperaturecompared to arc and microwave generated plasmas. The magnetic fieldresponsible for generating the plasma in radio-frequency plasma isnon-uniform in profile which leads to an uneven temperature gradient andthus a non-homogenous thermal treatment of the particles. This leads tonon-homogeneity in size, microstructure, and density or porosity of thefinal product.

Thus there is a need to provide a homogenous and uniform hightemperature thermal path for all the feed materials processed whichresults in high purity, contamination-free, and homogenous sphericalparticles. However, no such method has been reported.

From the above, it is therefore seen that there exists a need in the artto overcome the deficiencies and limitations described herein and above.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome and additional advantagesare provided through the use of a microwave generated plasma torchapparatus that is capable of producing laminar flow patterns tospheroidize and densify geometrically non-uniform powder particles andsolution precursor droplets of ceramic materials.

In accordance with one embodiment of the present invention a pressurizedpowder feeder is used to axially inject powder particles into a plasmachamber where the powder particles are entrained in a laminar gas flowpattern and undergo uniform heat treatment by being exposed to a uniformtemperature profile within the microwave generated plasma. This resultsin spheroidal pearl-like particles having uniform density.

In another embodiment of the present invention, a droplet maker oratomizer is used to inject solution precursor droplets which areentrained in a laminar gas flow pattern and undergo uniform heattreatment by being exposed to a uniform temperature profile within themicrowave generated plasma to produce spheroidal pearl-like particleshaving uniform density.

Another feature of this invention is that it uses microwave generatedplasma in accordance with U.S. patent application Ser. No. 13/445,947.

Therefore, an object of the present invention is to provide a laminarflow environment, free of turbulent flow effects, for the feed materialthat is processed with the microwave generated plasma, which results indense and spheroidal particles having uniform sizes and shapes andcharacterized by a homogenous materials distribution.

It is another object of the present invention to enhance plasmaprocessing of materials so as to provide a product with improved thermalproperties, improved corrosion and wear resistance and a highertolerance to interface stresses.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention.

The recitation herein of desirable objects which are met by variousembodiments of the present invention is not meant to imply or suggestthat any or all of these objects are present as essential features,either individually or collectively, in the most general embodiment ofthe present invention or in any of its more specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with the further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 illustrates a preferred embodiment of the apparatus used formaking dense and spheroidized particles which uses the microwavegenerating source described in US Patent Application Publication2008/0173641 A1, a dielectric plasma torch as described in U.S. patentapplication Ser. No. 13/445,947, and a materials feeder such as a powderfeeder, or solution precursor reservoir which dispenses powderparticles, or solution droplets, in accordance with the presentinvention.

FIGS. 2A and 2B illustrate densified and spheroidized MgO—Y₂O₃ particlesobtained by the microwave plasma process after injection of spray-driedpowder;

FIG. 3 illustrates a method of atomizing droplets using a nebulizingapparatus;

FIG. 4 illustrates densified and spheroidized 7% by weight Y₂O₃—ZrO₂(7YSZ) ceramic particles, 20 to 38 micrometers in diameter afterinjection of 7YSZ precursor droplets using a nebulizing injector;

FIG. 5 illustrates densified and spheroidized 7YSZ ceramic particles, 38to 53 micrometers in diameter after injection of 7YSZ precursor dropletsusing a nebulizing injector;

FIG. 6 illustrates densified and spheroidized 8YSZ ceramic particles, 15to 35 micrometers in diameter after injection of 9YSZ precursor dropletsusing a droplet maker;

FIG. 7 illustrates a modified embodiment consisting of an additionalprocess plasma gas to control the velocity of feed material and theirresidence time in the plasma zone;

FIG. 8. Illustrates a modified embodiment consisting of adding apassivation feed material in the form of a gas or mist through thecentral tube or the middle tube of the plasma torch;

FIGS. 9A and 9B illustrate a modified embodiment consisting of theaddition of one or many inlets to introduce passivation feed material tocoat spheroidized product particles after the plasma zone.

FIGS. 10A and 10B illustrate densified and spheroidized blendedalumina-titania and cobalt oxide ceramic particles, 10 to 50 micrometersin diameter after injection of corresponding blended spray dried powder;

FIG. 11 illustrates a method of spheroidization by injection of solidpowder using a powder feeder;

FIG. 12 illustrates a method of spheroidization by injection of a mistof droplets using a nebulizing nozzle; and

FIG. 13 illustrates a method of spheroidization by injection of acontinuous stream of uniform droplets using a frequency driven dropletmaker.

DETAILED DESCRIPTION

Referring to FIG. 1, an apparatus to produce dense and spheroidalproducts which includes a microwave radiation generator 1, a plasmachamber 2, a dielectric sheathing plasma torch 3, and a powder feeder,or solution precursor injector, 4. The microwave radiation generator 1,described in US Patent Application Publication 2008/0173641 A1 iscombined with plasma chamber 2 and plasma sheathing torch 3. Both 2 and3 are described in U.S. patent application Ser. No. 13/445,947. Theparticle feeder 4 is an injection apparatus with a pressurized source 5that can feed solid powder particles 6 into dielectric plasma torch 3.When powder injection is used, particles 6 may be a product ofspray-drying techniques or other techniques. Alternatively, particles 6may be droplets of solution precursor injected using an atomizer, or adroplet maker energized with a high frequency electrical drive.Pressurized sources 7 and 8 are used to introduce process gases asinputs into 3 to entrain and accelerate particles 6 along axis 11towards plasma 14. First, particles 6 are accelerated by entrainmentusing core laminar gas flow 10 created through annular gap 9. Coolinglaminar flow 13 created through annular gap 12, flows at not lower than100 standard cubic feet per hour in the case of solid powder feed oratomized injection, and provides laminar sheathing for the inside wallof dielectric torch 3 to protect it from melting due to heat radiationfrom plasma 14. High flow is also needed to keep particles 6 fromreaching the inner wall of 3 where plasma attachment could take place.Relatively lower gas flows are needed when using a droplet makerinjector as the flow of particles is more uniform and follows axis 11closely. Particles 6 are guided by laminar flows 10 and 13 towardsmicrowave plasma 14 were they undergo homogeneous thermal treatment tobecome dense and spherical product particles 15. The densification andspheroidization of spray-dried ceramic solid particles 6, or of dropletsof solution precursor materials, is achieved by choosing the appropriateexperimental parameters capable of maintaining stable microwavegenerated plasma 14 to produce dense and spherical particles 15. Theseparameters are microwave power in 1, powder particles or solutionprecursor droplet injection flow rates along axis 11, carrier gas flowrates of laminar entrainment flow 10, laminar cooling flow 13 inside thedielectric sheathing torch 3, heating rates within plasma 15 andquenching rates not less than 10³° C./sec upon exit of plasma 15.

Referring to FIGS. 2A and 2B, this method has been applied tospheroidize particles 6 made of commercial Magnesia-Yttria (MgO—Y₂O₃)solid powder particles obtained by the spray-drying technique from theINFRAMAT Corporation. Particles 6, in the case of INFRAMAT powder,possess closed or semi-spherical morphology, low density, and are highlyporous and brittle particles. The powder feeder 4, with a reservoir withlow pressure gas flow (<20 PSI) from pressurized source 5, provides afluidized bed for particles 6 which are driven by gas flow throughpowder feeder 4 towards the input of plasma torch 3. Particles 6diameters initially ranged between 38 micrometers (μm) and 73 μm. Theelimination of small particles prior to injection (particles withdiameters less than 37 μm) reduces any recirculation of materials abovethe hot zone. The elimination of large particles (larger than 75 μm)reduces the diameter range of particle products that will be collected.Sieves with mesh size 400 (38 μm), and 200 (74 μm) have been used toaccomplish this classification. Particles 6 are then entrained andaccelerated along axis 11 by laminar gas flow 10 for a minimum distanceof two (2) inches towards microwave plasma 14. Laminar flow 10 iscrucial in constraining the flow paths of particles 6 to a cylindricalregion as close as possible to axis 11. The penetration into plasma 14is accomplished in such a manner that directional paths of particles 6take place at the center of plasma 14 along axis 11. Particles 6 areagain accelerated, in part, by a second laminar gas flow 13, over aminimum distance of three quarters of an inch before reaching the top ofthe plasma flame 14. The primary function of laminar flow 13 is toensure adequate cooling of the dielectric tube sheath 3 that houses theplasma. Laminar flow 13 need to be high enough to span the remainder ofthe length of the outer tube of dielectric plasma torch 3. Theprocessing medium that combines the axis-symmetric laminar gas flows 10and 13, along with the uniform temperature profile of plasma 14, ensuresthat thermal processing of particles 6 is done volumetrically to yieldthe dense spherical products 15 shown in FIG. 2A. Two large ticks of theruler in FIG. 2B correspond to 100 micrometers which demonstrates thatparticles 15 range in diameter from 15 micrometers to 50 micrometers.The densified and spheroidized particles 6 exhibit a “pearl” liketexture and morphology.

Referring to FIG. 3, particles 6 are produced using a nebulizingapparatus 16 for solution precursor injection. Nebulizer 16 consists oftwo concentric quartz tubes 17 and 18 which are fused together. Asolution precursor of 7YSZ, or any other weight concentration of Yttriumranging from 3% to 20%, in a pressurized stainless steel tank isintroduced at input 19 of tube 17. The solution precursor injection flowrate is in the order of 4 milliliters per minute and the gas tankpressure is about 20 pounds per square inch (PSI). The length of tube 18is no smaller than 2 inches and does not exceed one foot. Through inputtube 20, a pressurized gas source 21 pushes the atomizing gas flow 22through an annular gap between concentric tubes 18 and 17 in nebulizer16. The injected solution precursor exits through orifice 23 where it isatomized by gas flow 22, and exits at orifice 24 of tube 18 as anaerosol of droplet particles 6. The distance between orifice 23 andorifice 24 must not exceed 1 millimeter (mm). The end of tube 18 istapered so that gas flow 22 enters the jet of solution precursor with anangle close to 90 degrees. Upon exit, particles 6 are entrained bylaminar flow 10 in dielectric plasma torch 3, also seen in FIG. 1, andsubsequently the particles reach the axis-symmetric thermal processingmedium inside dielectric plasma torch 3 where they undergo volumetricheating in plasma 14.

Referring to FIG. 4 and FIG. 5, these illustrate densified andspheroidized 7YSZ product particles 15 by atomizing 7YSZ solutionprecursor using nebulizer 16. Particles 15 exhibit a “pearl” liketexture and morphology. Particles 15 measure approximately between 20micrometers (μm) and 53 micrometers (μm) in diameter as shown in FIG. 4(20 to 38 μm), and FIG. 5 (38 to 53 μm) after post-classification withsieves having mesh sizes of 635 (20 μm), 400 (38 μm), and 270 (53 μm),respectively.

In yet another embodiment, a different feeder than a nebulizer can beused to produce atomized precursor droplets. In this case, a piezodriven droplet maker is used to generate a stream jet of uniformdroplets of 8%-weight YSZ precursor which are then injected into theplasma hot zone of the microwave plasma apparatus described previouslyin FIG. 1. This droplet maker apparatus is described in patentapplication published under number WO 2014052833 A1. Referring to FIG.6, spheroidized and densified 8YSZ product particles are produced with asize ranging from 15 to 35 micrometers as measured using an opticalmicroscope. A scale of 50 micrometers is included in FIG. 6. Thesespheres are pearl like indicating the presence a smooth surface forspherical and dense particles thus enhancing the flowability of suchpowder.

In yet another embodiment, an entraining gas is added to the centralfeeding tube so as to entrain the powder particle matter into the plasmaand control the powder particle matter velocity and residence time inthe plasma. The entraining gas can be chosen to be compatible with thepowder particle matter processing goal. For instance, if the materialbeing processed needs to be kept from oxidizing or being reduced, thenan inert gas such as argon can be chosen. If the material to bespheroidized is to be oxidized, then oxygen gas may be used, and if thematerial to be spheroidized is to be reduced, then hydrogen or ammoniacan be used.

Referring to FIG. 7, a pressurized source 25 is used to introduceadditional entrainment gas into tube inlet 26 in communication with exitnozzle 27 of powder feeder 4 containing feed particles 6 which areaccelerated inside central tube 28 of plasma torch 3. The flow rate ofinjected gas 25 is determined according to the size and morphology ofinjected particle powders to optimize the melting conditions of theseparticles in the plasma zone. It is well known by those skilled in theart that additional gas 25 can be injected so that it is collinear withthe direction of flow of feed particles 6 by an obvious change in themanner of communication of inlet 26 with powder feeder exit nozzle 27and central tube 28 of plasma torch 3.

In yet another embodiment, passivation-inducing fluid is added into theprocess so as to passivate and/or coat the spheroidized powder. Thepassivation fluid can be either gaseous or atomized solution. Referringto FIG. 8, pressurized source 25 is used to introduce passivation fluid29 in the form of a gas phase into inlet tube 26 in communication withexit nozzle feeder 27 of powder feeder 4 which contains feed particles6. Referring to FIG. 8 again, the passivation materials 29 can beintroduced as a mist using atomizing gas source 30 and nebulizer 31 intoinlet tube 26 in communication with exit nozzle feeder 27 of powderfeeder 4 which contains feed particles 6. The mixture of passivationmaterial and feed particles travels together in central tube 28 towardsthe plasma zone where the passivation material chemically reacts withfeed particles 6.

Referring to FIG. 8, the passivation-inducing fluid 32 can also be addedthrough inlet tube 33 in communication with middle tube 34 of plasmatorch 3. Passivation fluid 32 is introduced as gas using pressurizedsource 35 or as mist using a combination of atomizing gas 36 andnebulizer 37. The mixture of passivation material 32 and feed particles6 are mixed together in the middle tube 34 at the outlet end of centraltube 28 and entrained together towards the plasma zone where thepassivation material chemically reacts 32 with feed particles 6.

In yet another embodiment, the passivation fluid is injected into theprocess through an additional injection port, located after themicrowave injection port, i.e., downstream the plasma hot zone. In thissituation, the powder is first spheroidized, and then exposed to thepassivation fluid that can be in the form of a gas or atomized solution.The object of this invention is to provide means to produce core-shelllike microstructure of spheroidized particle products. Referring to FIG.9A. Passivation fluid material is injected as a gas phase or atomizeddroplets into inlet 38 inside plasma gas exhaust chamber 39 so thatpassivation fluid material is deposited on plasma processed particle 15to produce coated particle 40. Passivation fluid material can beintroduced as a gas or atomized droplets using the means described. Asecond tube inlet 41 and even a multitude of inlet tubes can be used tointroduce the same passivation fluid material or a combination ofdifferent coating fluid materials. It is obvious for someone skilled inthe art that the number of these inlets into exhaust chamber 39 can beincreased to accommodate a composite mixture for the desired passivationor coating layer depending on the applications. Referring to FIG. 9B, amagnified passivated or coated particle 40 is shown with thespheroidized core 15 and shell-like passivation coating 42.

This method has been applied to spheroidize particles made of a blend ofcommercial alumina-titania and cobalt oxide powders shown in FIG. 10A.This blend consists of 80%-weight alumina-titania (Al₂O₃-13TiO₂) and20%-weight cobalt oxide (CoO) agglomerated powder particles obtained bythe spray-drying technique averaging 37 micrometers in diameter. Thewhite graduated scale (ruler) in the background indicates 100micrometers between two long graduated bars, while the distance is 10micrometers between two consecutive small graduation bars. FIG. 10Bshows spheroidized and densified product particles after processingthrough the microwave plasma. It can be seen that most of the productparticles exhibit a pearl-like texture with a diameter size rangingbetween 10 and 50 microns.

The electrodeless microwave generated plasma can be used to purify awide range of materials. Presence of electrodes to generate plasma, suchas arc plasma processes, introduces impurities during in-flight meltingof processed materials. Impurities are made of electrode material as theelectrode wear out due to high operating temperatures of arc plasmas.Absence of electrodes to generate microwave plasma used in the presentmethod eliminates contamination and provides for the production of highpurity processed materials.

Referring to FIG. 11, the densified and spheroidized particles are madeaccording to the procedure described therein. The powder particles to beprocessed are first sifted and classified according to their diameters,the minimum diameter is 20 μm and the maximum diameter is 74 μm. Thiseliminates recirculation of light particles above the hot zone of theplasma chamber and also ensures that the process energy present in theplasma is sufficient to melt the particles. This powder is then disposedin a powder feeder where a fluidized bed, in an internal chamber using arelatively low pressurized source of air not exceeding 20 pounds persquare per inch (PSI), is introduced. For a better flow of the powder,the powder feeder is constantly shaken using a shaker energized byanother pressurized source of air having a minimum pressure of 20 poundsper square per inch (PSI). The powder is carried from inside the powderfeeder towards the input of the feeding tube of the dielectric plasmatorch under pressure which permits a constant injection of particlesinto the plasma process. Prior to this, and referring to the right sideof FIG. 11, microwave radiation is introduced into the waveguide towardsthe plasma chamber where the dielectric plasma torch is located, andplaced perpendicularly to the waveguide. Two annular flows areintroduced; one for entrainment of injected particles and the other flowprotects the inner wall of the outer tube of the plasma torch frommelting under the effect of the high heat from the plasma. Once bothflows are in place, the plasma is ignited inside the dielectric plasmatorch. An appropriate combination of entrainment and cooling flows arechosen to stabilize the plasma. In addition, these entrainment andcooling flows are chosen to allow smooth circulation of particlestowards the plasma and avoid turbulence that could create recirculationand back flow of powders above the hot zone of the chamber. In additionthese entrainment and cooling flows are chosen to minimize anynon-uniformity in the thermal path in the outward radial direction awayfrom axis 11. Once the particles reach the plasma now present in the hotzone, they are subjected to a uniform melt state characterized by auniform thermal path for particles along with a uniform temperatureprofile of the plasma in the hot zone. The particles are processedvolumetrically and uniformly and exit into an atmospheric fast quenchingchamber below the exit nozzle of the plasma. The product uponsolidification is collected in nylon or stainless steel filters, orquenched in distilled water in some applications, and analyzed for itsmicrostructure and its mechanical, optical, and thermal properties.

Referring to FIG. 12, the figure illustrates a procedure to producedensified and spheroidized particles using solution droplets. Thedesired chemical composition is first mixed according to the assignedproportions of reactants. Subsequently it is thoroughly stirred to yielda homogenous molecular mix of reactants. The solution is then pouredinto a stainless steel tank. A pressurized source of air is used toinject air into the tank and push the solution towards an injectornozzle similar to a nebulizer, and injected into the central feedingtube of the nebulizer where it emerges as a jet. At the same time,another pressurized source is used to push air into an outer concentrictube of the nebulizer which penetrates the solution jet perpendicularly.Consequently this atomizes the jet into a mixture of droplets ofdifferent size diameters which are directed towards the plasma. Prior tothis, referring to the right side of FIG. 12, the procedure to ignitethe plasma is repeated as described in the paragraph referring to FIG.11. Microwave radiation is introduced into the waveguide towards theplasma chamber where the dielectric plasma torch is located, and placedperpendicularly to the waveguide. Two annular flows are introduced; onefor the entrainment of injected particles and the other flow to protectthe inner wall of the outer tube of the plasma torch from melting underthe effect of the high heat from the plasma. Once both flows are inplace, the plasma is ignited inside the dielectric plasma torch.Adequate combination of entrainment and cooling flows are chosen tostabilize the plasma. Also, these flows are chosen so as to allow asmooth circulation of droplets towards the plasma and avoid turbulencethat could create recirculation and back flow of powders above the hotzone of the plasma chamber, as well as avoiding a disruption in thethermal path. Once the droplets reach the plasma now present in the hotzone, they are subjected to a uniform melt state characterized by auniform thermal path along with a uniform temperature profile of theplasma in the hot zone. The droplets are processed volumetrically anduniformly as solvent materials produced by oxidization and reductionreactions are burnt off. The processed particles exit into anatmospheric fast quenching chamber below the exit nozzle of the plasma.Upon solidification, the product is collected in filters, or in someapplications quenched in distilled water, and analyzed for itsmicrostructure and its mechanical, optical, and thermal properties.

Referring to FIG. 13, the figure illustrates a procedure to producedensified and spheroidized particles using uniform solution precursordroplets produced using a droplet maker. The desired solution's chemicalcomposition is prepared by first mixing the assigned proportions ofreactants. Subsequently, the solution is thoroughly stirred to yield ahomogenous molecular mix of reactants. The solution is then pumpedinside the reservoir of a droplet maker by means of a peristaltic pump,or a pressurized tank. Once the reservoir is full, a piezo transducer isactivated which impinges an adequate perturbation onto the liquidsolution in the reservoir. Once the perturbation satisfies Rayleigh'sbreakdown law, the solution emerges through a capillary nozzle as acontinuous stream of uniform droplets exiting at a constant speed for agiven drive frequency of the piezo. The nature of the droplets stream ismonitored so that it is not in a burst mode or incidental mode but inthe form of a jet with uniform droplets. This stream of droplets is theninjected into the feeding tube of the dielectric plasma torch where itundergoes the same plasma process, and subsequently is transformed intoa collection of dense and spheroidized particles as described in theparagraphs referring to FIG. 11 and FIG. 12.

EXAMPLES Example 1 Spheroidization of MgO—Y₂O₃ Spray Dried Particles

Referring to FIGS. 2A and 2B, this method has been applied tospheroidize particles made of commercial Magnesia-Yttria (MgO—Y₂O₃)solid powder particles obtained by the spray-drying technique from theInframat Corporation. The densified and spheroidized particles exhibit a“pearl” like texture and morphology.

Example 2 Spheroidization of Atomized 7YSZ Solution Precursor

Referring to FIG. 4 and FIG. 5, this method has been applied to producedense and spheroidized particles directly from the injection of atomizeddroplets of 7%-weight Yttria-Stabilized-Zirconia (7YSZ) solutionprecursor using a nebulizer.

Example 3 Spheroidized of a Stream Jet of Uniform Droplets of 8YSZSolution Precursor

Referring to FIG. 6, this method has been applied to produce dense andspheroidized particles directly from the injection of uniform dropletsof 8%-weight Yttria-Stabilized-Zirconia (8YSZ) solution precursor usinga high frequency piezo driven droplet maker.

Example 4 Spheroidization of Blended Alumina-titania Oxide(Al₂O₃-13TiO₂) and Cobalt Oxide (CoO) Powder Particles

Referring to FIG. 10, this method has been applied to spheroidizeparticles made of a blend consisting of 80%-weight alumina-titania(Al₂O₃-13TiO₂) and 20%-weight cobalt oxide (CoO) agglomerated powderparticles obtained by the spray-drying technique. The densified andspheroidized particles exhibit a “pearl” like texture and morphology.

What is claimed is:
 1. A method of producing spheroidal and dense powderproduct particles extracted from plasma exhaust gas following injectionof feed materials consisting of powder particles or solution precursordroplets using a microwave plasma and which method comprises:introducing axially feed material through a materials feeder into anaxisymmetric microwave plasma torch; entraining said feed material usingan axis-symmetric laminar entrainment flow; guiding said feed materialtowards the microwave plasma using an axis-symmetric laminar coolingflow; processing said feed material by exposing it to high temperatureprofile within the microwave generated plasma; filtering said exhaustgas of said microwave generated plasma; and extracting powder productparticles from said filtered exhaust gas.
 2. The method of claim 1wherein said feed material is a powder with a substantiallynon-spherical shape.
 3. The method of claim 1 wherein said feed materialis a solution or a suspension of solution precursors injected asdroplets.
 4. The method of claim 1 wherein said materials feeder uses afluidized bed.
 5. The method of claim 1 wherein the materials feeder isa nebulizing atomizer which produces a mist of droplets having adiameter size ranging between 0.5 micrometers and 200 micrometers. 6.The method of claim 1 wherein the materials feeder is a droplet makerwhich uses a piezo electric transducer that injects uniform droplets. 7.The method of claim 1 wherein the filtering step includes filtering saidplasma exhaust gas.
 8. The method of claim 1 wherein the filtering stepincludes collecting powder product particles in water.
 9. The method ofclaim 1 wherein the powder product particles are spherical.
 10. Themethod of claim 1 wherein the powder product particles are densified toa range of 95% to 100%.
 11. The method of claim 1 wherein the plasma gasis inert.
 12. the method of claim 1 wherein said plasma gas has anadditional component injected with said feeding material.
 13. A methodof producing passivated spheroidal and dense powder product particlesextracted from plasma exhaust gas following injection of feed materialsconsisting of powder particles along with passivation material using amicrowave plasma and which method comprises: introducing axially feedmaterial through a materials feeder into an axisymmetric microwaveplasma torch; entraining said feed material using an axis-symmetriclaminar entrainment flow; guiding said feed material towards themicrowave plasma using an axis-symmetric laminar cooling flow;introducing passivation material next to said feed material; processingsaid feed material and said passivation material in a uniformtemperature profile within the microwave generated plasma; filteringsaid exhaust gas of said microwave generated plasma; and extractingpowder product particles from said filtered exhaust gas.
 14. The methodof claim 13 wherein said passivation material is a material in a gaseousphase.
 15. The method of claim 13 wherein said passivation material isan atomized liquid material.
 16. The method of claim 13 wherein saidproduct particles are spherical and have a shell-core structure.
 17. Themethod of claim 16 wherein the core structure is made out of the feedmaterial.
 18. The method of claim 16 wherein the shell structure is madeout of the passivation material.
 19. A method of producing passivatedspheroidal and dense powder product particles extracted from plasmaexhaust gas following injection of feed materials consisting of powderparticles along with passivation material using a microwave plasma andwhich method comprises: introducing axially feed material through amaterials feeder into an axisymmetric microwave plasma torch; entrainingsaid feed material using an axis-symmetric laminar entrainment flow;guiding said feed material towards the microwave plasma using anaxis-symmetric laminar cooling flow; processing said feed material in auniform temperature profile within the microwave generated plasma;introducing said passivation material into plasma exhaust pathdownstream from microwave energy input; filtering said exhaust gas ofsaid microwave generated plasma; and extracting said coated powderproduct particles from said filtered exhaust plasma gas.
 20. The methodof claim 19 wherein said passivation material is a material in a gaseousphase.
 21. The method of claim 19 wherein said passivation material isan atomized liquid material.
 22. The method of claim 19 wherein saidproduct particles are spherical and have a shell-core structure.
 23. Themethod of claim 22 wherein the core structure is made out of the feedmaterial.
 24. The method of claim 22 wherein the shell structure is madeout of the passivation material.
 25. The method of claim 1 wherein theplasma gas is reducing.
 26. The method of claim 1 wherein the plasma gasis oxidizing.